(Multi-Layer Film and Brightness Enhancement Film)
A film which a large number of a layer having a low refractive index and a layer having a high refractive index are alternately laminated can be formed into an optical interference film which selectively reflects or transmits light having a specified wavelength due to structural optical interference between the layers. In addition, such a multi-layer laminate film is able to obtain a high reflection comparable to a film using a metal by gradually changing the film thickness or sticking a film having a different reflection peak and can also be used as a metal luster film or a reflection mirror. Furthermore, by stretching such a multi-layer laminate film to only one direction, the stretched film can reflect only a specified polarized light component, and it can also be used as a polarization reflection film. Therefore, it is known that by using such a multi-layer laminate film for liquid crystal displays or the like, it can be used as a brightness enhancement film for liquid crystal displays or the like (see PTLs 1, 2, 3, and 4, etc.).
(Polarizing Plate)
A liquid crystal display device (LCD) that is used for television receivers, personal computers, cellular phones, and the like makes it possible to serve as a display through regulating transmission of light outgone from a light source by using a liquid crystal panel composed of a liquid crystal cell and polarizing plates disposed on both sides thereof. As the polarizing plates stuck to the liquid crystal cell, an absorption type polarizing plate which is called a light-absorption type dichroic linear polarizing plate is generally used. A polarizing plate in which iodine-containing PVA is protected with triacetyl cellulose (TAC) is widely used.
Such an absorption type polarizing plate transmits polarized light in a direction of transmission axis and absorbs almost all of polarized light in a direction orthogonal to the transmission axis. Therefore, it is pointed out that about 50% of the light outgone from a light source device is absorbed by this absorption type polarizing plate, and light utilization efficiency is lowered. Then, in order to use effectively the polarized light in the direction orthogonal to the transmission axis, a configuration in which a reflective polarizer called a brightness enhancement film is used between the light source and the liquid crystal panel has been investigated. As an example of such a reflective polarizer, a polymer-type film that employs the above-described optical interference has been investigated (PTL 5, etc.).
On the other hand, regarding the polarizing plate to be stuck to the liquid crystal cell, various kinds of lamination configurations, inclusive of a reflection display that uses outside light, a transmission display that uses backlight, and the like, in which an absorption type polarizing plate and a reflection type polarizing plate are used in combination, have also been investigated in accordance with kinds and purposes of light used in the display device.
For example, PTL 6 discloses a liquid crystal display device in which an electric field is applied to the liquid crystal layer to vary a retardation value of the liquid crystal and shift a retardation value of the incident polarized light on the liquid crystal layer by a certain amount. As an example of the polarizing plates used on the both sides of the liquid crystal layer, PTL 6 discloses a reflection type polarizing plate of a planar multi-layer structure in which three or more films having birefringence are laminated on the light source side, and an absorption type polarizing plate provided on the opposite side relative to the liquid crystal layer.
In addition, PTL 7 proposes that on the occasion of using an absorption type polarizing plate and a reflection type polarizing plate as polarizing plates to be disposed on the both sides of a liquid crystal cell sandwiching a liquid crystal between flexible substrates, in order to dissolve warping generated due to a difference in the extent of temperature-dependent expansion and contraction between the respective polarizing plates, these polarizing plates are combined to form a specified laminate configuration, thereby dissolving the warping. Then, PTL 7 describes that a birefringent dielectric multi-layer film is used as an example of the reflection type polarizing plate, and specifically discloses a brightness enhancement film.
(Interlayer Adhesion)
However, some of the reflective polarizing polymer films using a multi-layer structure, which have hitherto been investigated, are not sufficient in adhesion between the multi-layer polymers, resulting in a problem that on processing or the like, the multi-layer part causes exfoliation.
For example, in the case of a multi-layer laminate film using polyethylene-2,6-naphthalenedicarboxylate (hereinafter sometimes referred to as “2,6-PEN”) for a high refractive index layer and polyethylene naphthalate (PEN) copolymerized with a thermoplastic elastomer or terephthalic acid in an amount of 30 mol % for a low refractive index layer as described in PTL 2 and the like, a difference in refractive index between the layers in a uniaxial stretched direction (X direction) is increased to enhance the reflectance of P-polarized light, whereas a difference in refractive index between the layers in a direction (Y direction) orthogonal to the in-plane direction of the film to enhance the transmission for S polarized light, thereby revealing a certain level of polarization performance.
However, according to the above-described combination of polymers, on the occasion of post-working the multi-layer laminate film, exfoliation is liable to be generated in the multi-layer part due to a cause of application of a stress, or the like. Thus, an enhancement in interlayer adhesion has been desired.
In addition, as a reflective polarizing polymer film having a multi-layer structure capable of being used as a reflective polarizing plate with a higher polarization degree, in PTL 8, the present inventors have investigated a reflection type polarizing plate capable of being used as a polarizing plate adjacent to a liquid crystal cell and composed of a multi-layer structure replaceable for the absorption type polarizing plate and proposed a film in which some specified polymer is used for the high reflective index layer and which when uniaxially aligned, enhances the polarization performance as compared with the conventional reflection type polarizing plates of a multi-layer structure.
However, though the reflective polarizing film proposed in PTL 8 realizes a high polarization degree of around 97 to 98%, there is a case where it may not be said that the interlayer adhesion is sufficient, and hence, a more improvement is demanded.
Then, an object of the present invention is to provide a uniaxially stretched multi-layer laminate film that is a polymer film of a multi-layer structure using a polyethylene naphthalate-based polymer for a layer having high refractive index characteristics, in which not only a certain level of polymerization performance is provided, but also the interlayer adhesion is improved.
(Prism Layer-Provided Reflective Polarization Type Brightness Enhancement Film)
With the spread of cellular phones, the needs for more thinning of display terminals are increasing, and thinning of members installed in the interior of display, such as a reflective polarization type brightness enhancement film, a diffusion film, a prism film, a light guide plate, etc., is demanded. Furthermore, a complex optical film capable of unifying these functions is desired. As an example of such a complex functional film, a prism layer-provided reflective polarization type brightness enhancement film in which a prism structure is formed on a reflective polarization type brightness enhancement film is proposed, as described in PTL 9 and the like.
(Prism Layer Adhesion)
However, when the prism layer is formed on such a reflective polarization type brightness enhancement film, the adhesion is possibly insufficient, and on the occasion of cutting the resulting reflective polarization type brightness enhancement film and installing it in a display terminal, there was a concern that breakage or exfoliation of the prism layer is generated.
On the other hand, in general, in a prism film, a prism layer is formed on a PET film base material. As for a general technique for forming a prism layer, the prism layer is formed by transferring a solvent-free UV curable resin onto the PET film base material. On that occasion, a coating layer is provided on the PET film base material, thereby enhancing the adhesiveness. Such an easily adhesive coating layer is provided in a film forming process of the PET film, and a thermosetting crosslinking agent is added to a binder component, such as polyester-based resins, acrylic resins, urethane-based resins, etc., thereby enhancing the adhesion between the base material and the prism layer. Especially, in the solvent-free UV curable resin as in the prism layer, the enhancement in adhesion due to dissolution of the base material with an organic solvent cannot be expected, and hence, a coating film capable of ensuring the adhesion is extremely limited (see PTLs 10 and 11, etc.).
In the case of forming such an easily adhesive coating film on a reflective polarization type brightness enhancement film, a polyester resin other than PET is frequently used as the reflective polarization type enhancement film, and furthermore, in order to reveal reflective polarization performance, in many cases, a crystallization process is not provided in the film forming process, and therefore, the reactivity of the thermosetting crosslinking agent becomes low. Thus, even by using the same coating liquid composition as in the PET film base material, it was difficult to ensure sufficiently the adhesion to the prism layer.
In addition, in the reflective polarization type brightness enhancement film, a surface layer formed of a specified polyester resin other than PET is liable to be scarred, and hence, it was necessary to stick a protective film or the like during the process. The matter that such a protective film is not used is also demanded, and it was desired to enhance the adhesiveness of the prism layer to the reflective polarization type enhancement film serving as the base material and further to ensure winding-up properties and the like.
Then, a desired object of the present invention is to provide a uniaxially stretched multi-layer laminate film using a polyalkylene naphthalate-based polyester for a high refractive index layer, the multi-layer laminate film being provided with reflective polarization performance and high adhesion between a prism layer to be formed on the film and the film.
PTL 1: JP-A-4-268505
PTL 2: JP-A-9-506837
PTL 3: JP-A-9-506984
PTL 4: WO01/47711
PTL 5: JP-A-9-507308
PTL 6: JP-A-2005-316511
PTL 7: JP-A-2009-103817
PTL 8: JP-A-2012-13919
PTL 9: JP-A-9-506985
PTL 10: JP-A-2008-36868
PTL 11: JP-A-2008-189868